Images for which previews can't be generated can't be viewed

[icewind1991]

Describe the bug

Images for which previews can't be generated can't be viewed, it will show an error messages with "Error loading ..." instead

To Reproduce Steps to reproduce the behavior:

1. Upload an image file to large to generate a preview for (~6000x6000) or set preview_max_memory to a value lower than the default 128.
2. Try to view the image

Expected behavior

If a preview can't be generated, the viewer should fallback to loading the original image.